1. Field of the Invention
The invention relates to a method for the forming of a solder deposit on an SMD pad on a printed circuit or hybrid board. This formed solder deposit is in a defined three-dimensional well having the desirable profile and a defined solder gap. It is immaterial whether the solder before forming is solid solder or a solder paste. By the placement of a mesh on the surface of the circuit board with the solder in place on the pads, applying a slight positive pressure on a rigid or elastic surface on the other side of the mesh, and subjecting this system to a temperature just low enough to reflow the solder by means of a heat transfer fluid either liquid or gaseous, then cooling the board to solidify the solder, a product results having the above properties. The invention also relates to apparatus for forming this solder deposit as well as a circuit board with the solder so formed thereon.
2. The Prior Art
Various methods are known for applying solder selectively to printed circuit boards (PCBs) in such a way that following the assembly of the boards with Surface Mount Device (SMD) components, the boards can be electrically and mechanically connected to the components by reflow soldering methods.
In particular, the prior art uses a method in which the solder deposit is applied to the selected regions of the PCB in the form of a deposit of solder by screen printing or stencil printing with soldering pastes. While this method is in common use today, nevertheless, in many respects, this method cannot be considered optimal:
1--the investment cost of screen printing machines, screens and masks or stencils required is considerable;
2--the screens or stencils that must be used wear regularly;
3--the entire screen printing process must be monitored continuously because it cannot be precluded that the screens used may become plugged in some regions;
4--so-called thin solder deposits are often produced, which can mean low or no mechanical and thermal load capacity for the solder connections based on them;
5--thixotropic properties change while pastes are exposed to variations in temperature, humidity, evaporation of solvent and shearing conditions;
6--soldering pastes are expensive;
7--the resolution of the screen printing method is limited; and
8--the optoelectronic positional recognition of assembly systems involves great difficulties not only because of solder paste deposits that do not properly match the intended contour resulting in deviations in production, but also because of movement of the PCB itself.
9--The flexibility of interlinked production lines having screen printing is greatly impaired because these machines entail considerable set-up times and adjustments.
10--Moreover, the heating speed of the introduction of heat in reflow soldering with solder pastes has proven to be limited, because in the heating process, volatile ingredients and solvents from the paste have to evaporate which takes time.
11--Other disadvantages associated with solder paste are: a)bubble formation, b)oxidation of the soldering paste, c)fine granulation and the like.
Another previously used method in the prior art is known as immersion application of solder to PCBs. In this method, a prepared PCB is dipped into and removed from a solder bath. After being removed and following a cooling down phase, the metallized regions of the PCB provided with a solder resist means are provided with a solder deposit, which, however, has a more or less convex cross-sectional shape; this rounded surface forms because of the high surface tension of solder. The height of the solder deposit is also dependent on the dimensions in the plane of the PCB of the regions to which solder is to be applied, so that when such regions have different dimensions, variably high solder deposits necessarily result.
U.S. Pat. No. 5,051,339 issued Sep. 24, 1991 Friedrich et al., the so-called "OPTIPAD" process, is an attempt to overcome some of these disadvantages. Because this is an immersion process, there are no foreign ingredients, that is, no ingredients but solder in the solder deposits.
The process, simply stated, involves procuring a PCB having thereon a solder mask with pads exposed, laminating thereto a photoimageable layer of perhaps 5 mil in thickness, exposing and developing so that everything is masked but the pads to be soldered, immersing the thus prepared PCB into molten solder, and then contacting the board with a closure element to maintain the solder in place until it solidifies. Optionally, the limiting layer, a photoimageable solder resist mask or a regionally applied foil layer, can be left on if the component has leads which must be connected to the pads or can be removed if SMD lead-less components are used.
This patentee has pointed out that the use of hot air or hot oil leveling is not considered to be close prior art, whereas immersion solder application without ensuing air leveling is.
Some of the specifics of this invention are as follows:
solder bath temperature=250 degrees C.; PA1 optionally apply flux prior to immersion; PA1 optionally apply a primary application of solder; PA1 optionally supply turbulence in the solder bath; PA1 optionally employ closure elements with elastic surfaces; PA1 closure elements must not be wettable by solder; PA1 closure elements may have a deformably embodied closure surface, pressed into the voids located above the regions to which solder is to be applied; PA1 optionally have different temperature zones for quenching. PA1 with respect to solder paste, screen printing has already reached its limit; PA1 inability of the paste to maintain its profile after printing, therefore PA1 all the peripheral problems associated with paste and screening; PA1 buried defects in reflowed soldered joints using paste which can only be seen by X-ray examination; PA1 these defects also due to failure of some components of the paste to fully evaporate during pre-heating or reflow, with PA1 consequent solder joints with reduced peel strength; PA1 requirement for highly activated fluxes;
The coating of the PCB with the temporary soldering top mask is considered to be an indispensible part of the invention. The reason the patent application was allowed to issue was that the closest prior art did not contain the step of covering, with at least one closure element at a defined contact pressure, the voids located above the regions to which solder is to be applied when filled in the soldering bath with liquid solder.
The "OPTIPAD" process requires the use of a 5 mil temporary coating which is imaged, developed where the pads are and then immersed in molten solder. Others are attempting to modify this process by screening the molten solder into the pad wells. Stripping the temporary layer leaves behind a 5 mil high pillar of solder. Aside from the need for expensive equipment which is not commercially available, this process has two major problems:
1--When the temporary coating sees molten solder it cures extensively and is difficult to remove even when stripping with caustic soda which has the other effects of not only dulling and oxidizing the solder, but of attacking the permanent mask.
2--When the 5 mil solder mask is stripped it leaves pillars of solder which in fine pitch applications when mated with their components collapse and generate extensive shorts.
3--Any attempt to reduce the thickness of the mask in order to reduce the height of these pillars would result in a greater degree of curing and an even more difficult stripping operation.
In addition to the '339 patent mentioned above, there are two other relevant publications; a paper by W. J. Maiwald of Siemens entitled, "Reliable Reflow Soldering Techniques using Preformed Solid Solder Deposits, Part 2--The Assembly Process" and the joint paper by M. Weinhold of DuPont entitled, ". . . Part 1--The Printed Circuit Fabrication Process".
The "SIPAD" process of Siemens combines a few known technologies and produces a flat solid solder deposit on the PCB. The processing steps are applying solder pastes onto boards with permanent solder masks, melting of the solder paste and flattening of the round, humped solder deposits by a thermal/mechanical process.
The major problems which both the "OPTIPAD" and the "SIPAD" processes have attempted to address are the following:
1--opens and shorts, squeezing out of solder and resultant solder bridging; this latter occurs at assembly in the "OPTIPAD" process, and while flattening in the "SIPAD" process
2--low packing density and inability to solder with high pin counts without extensive design modifications
3--the printing of solder paste:
4--achieving the required shape of the deposit;
5--presence of a "hump", the meniscus, makes positioning of fine pitch components difficult resulting in unacceptable tolerances;
6--solder balls;
7--shelf-life of solder joint; short storage times;
8--poor solderability due to too thin deposits. Hot Air Solder Leveling is notorious for leaving thin deposits, with consequent growth of an intermetallic phase preventing wetting of the SMD solder pads during reflow or wave soldering;
9--poorly defined soldering gap;
10--inability to quantify and standardize solder deposit and solder gap;
11--yield after soldering. First-pass yields in standard applications are running at 60-70%, while for many fine pitch cases they are only 10%; rework is extensive;
12--overall cost;
13--quality of board and solder joint.
The solutions and benefits both of these approaches have attempted to achieve are as follows:
1--removal of the solder paste printing process from the assembler's operation; the PCB fabricator can supply to the assembler circuits with a solid, flat solder deposit. New processes can be used to replace solder paste printing and which obviate the need for cleaning the PCB assemblies without the risk of solder balls or other contamination;
2--guaranteed reflow solderable PCBs;
3--unlimited good solderability of the PCB;
4--small solder structures;
5--a defined solder gap; solder in a defined three-dimensional well, i.e., an excellent profile;
6--assembler would then use a 100% tested presoldered board since faults arising from solder application can be separately controlled, eliminated or reworked at the PCB fabricator without the obstruction of components;
7--the problem of solder paste deposits being deformed when the component terminals touch down is non-existent with this method. Components can be placed on a flat surface which would permit the use of fine-pitch flat packs and TAB assembly with automated equipment;
8--finer PCB structures can be implemented;
9--practically no shorts;
10--no solder balls;
11--too thin solder layers are not encountered;
12--fluxing agents can be optimized;
13--unlimited shelf-life between placement and soldering; longer guaranteed storage time; reduction in growth of the intermetallic phase resulting in higher peel strength;
14--the possibility of quantifiable and standardized solder deposits and solder joints;
15--better yield after soldering with considerable reduction of rework; higher first pass yields;
16--better overall quality of boards and solder joints; improved product consistency;
17--lower cost due to faster throughput in assembly with shorter SMD assembly lines.
Neither of these two processes, i.e., the "OPTIPAD" nor the "SIPAD", have been completely successful. The "OPTIPAD" process is run in an extremely aggressive environment, that is, molten solder at 450 degrees F. As a consequence, the so-called limiting layer or temporary film is severely cured and cannot easily be removed even with strong caustic solution. In the worst case, the permanent mask is also damaged. Attempts to resolve this by screening molten solder have had not only the same problem but in addition there is severe dulling of the solder undoubtedly due to its oxidation. As a consequence, any solder joint formed therefrom would probably have a short storage life due to intermetallic phase formation. Furthermore, this process requires the development of unique and expensive equipment for its implementation.
"SIPAD" also requires highly specialized equipment although attempts are being made to run this process in a conventional multilayer press normally used for PCBs. The problem here is that this type of press requires about 2000 psi before activation, and though the platens see the top of the solder first, they then contact the PCB itself and thermally shock the laminate, oftentimes scorching and mechanically damaging the solder mask as well. Furthermore, the solder when compressed, squeezes sideways as a very thin film or foil. While this problem can be solved by various techniques to remove the film, it results in extensive and expensive rework and fine droplets of the thus squeezed out solder end up as solder balls.
On the other hand, the "SIPAD" process uses the solder in the well to solder not the pillar of "OPTIPAD"; the process of this invention uses both.
Since solder "wicking" can be either sideways or upwards, in order for Siemens to achieve their maximum densities on fine pitch, they must change the design of their pads by elongating them. Since they are a company completely integrated vertically, this can be done by their designers, not so with the majority of OEM's. Wicking upwards is, of course, desirable; sideways is not.
In summary, both of these new processes leave much to be desired and are not practical in their present state of development. As a consequence, the study which resulted in this subject invention was undertaken.